FIG. 1 schematically shows an integrated circuit comprising a prior art circuit 1 for distributing a reference voltage. In integrated circuits, for example in mixed-signal ICs, it is often felt the need of generating a precise and clean reference voltage VR, normally of the bandgap type, and then buffer and distribute it with different amplification factors 1+R1/R0, . . . , 1+RN/R0, thus obtaining a plurality of buffered and amplified reference voltages VR1, . . . , VRN, fed to a number of independent receiving circuits CIR1, . . . , CIRN belonging to the integrated circuit. Buffering is generally required in order to avoid that the load perturbs the circuit BGV_G adapted to provide the bandgap reference voltage VR.
The bandgap reference voltage VR has generally very low noise and is referred to a clean ground. In order to have minimum noise injection and/or best power supply rejection it is desired that also the buffered/amplified reference voltages VR1, . . . , VRN have low noise and are referred to their local and noisy grounds GND1, . . . , GNDN. For example, let's assume that one of the buffered/amplified reference voltages for VR1, . . . , VRN, for example VR1, is used as reference voltage for a receiving circuit in the form of an analog-to-digital converter CIR1 built on its proper local ground GND1. By definition, an analog-to-digital converter compares the input signal to be converted with the difference between its reference voltage VR1 and its local ground GND1. A man skilled in the field knows that the buffered/amplified reference voltage VR1 shall be noise free, otherwise the A/D conversion provided by CIR1 would be affected by its noise, and for the same reason the reference voltage VR1 shall also to be referred to its local ground GND1.
The prior art solution shown in FIG. 1 is affected by two main drawbacks. The first drawback is due to the fact that a relatively long path L, which usually does not have very low output impedance, can collect noise or disturbances which result into crosstalk or pollution among the different circuits CIR1, . . . , CIRN of the chip. The second more important drawback is due to the fact that the noise VnGNDi of each local ground i=1, . . . , N is amplified by a factor −Ri/R0 at the amplifier OAi output, as know by an expert of the field, thus the buffered and amplified reference voltage VRi is not properly referred to its local ground because of a (−Ri/R0−1) VnGNDi noise term which impacts the difference VRi−GNDi. This is therefore the ultimate limitation of the above described solution even if ideal and noise free amplifier and resistors are assumed.
According to the prior art, the first of the above indicated drawbacks can be solved or limited by building buffering and amplification circuits close to the bandgap voltage generation block BGV_G. However, this expedient does not help to solve the second of the above mentioned drawbacks.